1. Field of the Invention
The present invention relates to an interlayer film containing silica with specific refractive index to provide such film with improved mechanical properties at low haze.
2. Description of the Related Art
Interlayer films for laminated glazing are long known in the art and mostly produced from plasticized polyvinyl butyral by an extrusion process. Nearly all windshields for cars or airplanes are produced from laminated glazing; in addition laminated glazing is often used as safety glazing for architectural applications.
To control the adhesion of the interlayer film in laminated glazing, a lot of research has been done. Besides the use of special plasticizer or adhesion control agents, the addition of filler materials such as inorganic compounds, especially silica has been proposed.
U.S. Pat. No. 2,467,342 describes the addition of colloidal silica to polyvinyl acetal resins to cast films. The colloidal silica disclosed in this publication must be in sol form in order to produce a clear film without haze. These casting processes are not a useful for the industrial production of polyvinyl acetal films.
EP 0 227 633 discloses the use of silica as nucleating agent in interlayer films for fire resistant laminated glazing. This interlayer film comprises a plasticizer, an organic phosphate as a char-forming component, and silica as a nucleating agent. In order to prevent undesirable haze of the interlayer film due to the addition of silica, only silica having a refractive index within ±0.03 of the plasticized formulation can be used. The refractive index of the plasticized formulation depends largely on the plasticizer composition, which includes organic phosphates. Therefore only certain combinations of silica and plasticizer mixtures are usable and disclosed in this publication. Organic phosphates are no longer used as plasticizer due to environmental concerns.
Fumed silica like the disclosed Cab-O-Sil M5 show refractive indices nD of 1.460, whereas PVB-films using the common non-aromatic plasticizer in standard amounts show refractive indices nD close to 1.482 at 20° C. In order to avoid an unacceptable haze value, EP 0227633teaches to use only small amounts of silica (at most 2.5% by weight based on the film) and silica with a small difference in refractive indices with the PVB. The mechanical properties of interlayer films are nearly unaffected by the addition of such low amounts of silica. The addition of large amounts of silica according to EP 0 227 633 would lead to an unacceptable haze value due to the difference in refractive indices.
In addition, the refractive index of interlayer film varies with temperature, for example between 10° C. to 50° C., refractive indices nD between 1.486 and 1.470 can be measured. Laminated glazing should be transparent over a broad range of temperature. The disclosed combinations of silica and plasticizer are not suitable for applications which are subject to temperature changes like cars or buildings, since below 20° C., the difference in refractive indices between silica and the interlayer film will even increase. It is therefore important to match the refractive index of the fumed silica as much as possible with the refractive index of the interlayer film at moderate temperatures like 20° C. in order to balance the increase of haze at higher and lower temperatures.
EP 1 042 121 discloses the use of small amounts of fumed silica in polyvinyl butyral films to enhance the adhesion between interlayer film and glass. In order to maintain the haze of the interlayer film in an acceptable range, only 0.001 to 0.25% by weight of silica is added to the plasticized polyvinyl butyral. By adding such small amounts of silica, no effect on the mechanical properties such as the tensile strength of the interlayer film can be detected.
JP11-060290 discloses the addition of silica and other metal oxides to polyvinyl acetal in combination with the metal salt of a carboxylic acid with 5 to 25 carbon atoms. In this publication, an upper limit of 0.5% by weight of silica is proposed in order to increase the adhesion of an interlayer film to glass and at the same time maintain an acceptable haze value. If used in the disclosed amounts, silica has practically no effect on the mechanical properties of the interlayer film.
Most of the published literature addresses the problem that by adding silica to plasticized polyvinyl acetals, the resulting interlayer films will obtain an unacceptable haze value for optical applications. Whether this unwanted haze value is due to the silica particles themselves or due to higher moisture sensitivity remains unclear.
So far, only small amounts of silica have been used to increase the adhesion between glass and interlayer film if at the same time an opaque appearance of the interlayer film has to be avoided. Furthermore, if silica is used in higher amounts than those described by the above-mentioned publications, not only unacceptable haze values but also discoloration of the film may result due to the acid properties of the silica. Acidic conditions will lead to partial degradation of the acid sensitive PVB-polymer at the high temperatures used for PVB-film extrusion.